Common envelope: enthalpy consideration

TL;DR

This paper proposes a modification to the common envelope ejection criterion by incorporating enthalpy, which could resolve discrepancies in binary evolution models and improve understanding of envelope dispersal during stellar interactions.

Contribution

It introduces an enthalpy-based energy criterion for common envelope ejection, refining the standard $\alpha_{ m CE}\lambda$-prescription to better match observational data.

Findings

Enthalpy consideration significantly alters the energy balance during CE events.

The new criterion may reduce the need for unphysically high efficiency parameters.

Potentially improves binary population synthesis models for LMXBs.

Abstract

In this Letter we discuss a modification to the criterion for the common envelope (CE) event to result in envelope dispersion. We emphasize that the current energy criterion for the CE phase is not sufficient for an instability of the CE, nor for an ejection. However, in some cases, stellar envelopes undergo stationary mass outflows, which are likely to occur during the slow spiral-in stage of the CE event. We propose the condition for such outflows, in a manner similar to the currently standard $\alpha_{\rm CE}\lambda$-prescription but with an addition of $P/\rho$ term in the energy balance equation, accounting therefore for the enthalpy of the envelope rather than merely the gas internal energy. This produces a significant correction, which might help to dispense with an unphysically high value of energy efficiency parameter during CE phase, currently required in the binary population synthesis studies to make the production of low-mass X-ray binaries (LMXBs) with a black hole companion to match the observations.